Daniel W. Urish

The discharge of nitrate-contaminated groundwater from developed shoreline to marsh-fringed estuary

As residential development, on-site wastewater disposal, and groundwater contamination increase in the coastal zone, assessment of nutrient removal by soil and sedimentary processes becomes increasingly important. Nitrogen removal efficiency depends largely on the specific flow paths taken by groundwater as it discharges into nitrogen-limited estuarine waters. Shoreline salinity surveys, hydraulic studies, and thermal infrared imagery indicated that groundwater discharge into the Nauset Marsh estuary (Eastham, Massachusetts) occurred in high-velocity seeps immediately seaward of the upland-fringing salt marsh. Discharge was highly variable spatially and occurred through permeable, sandy sediments during low tide. Seepage chamber monitoring showed that dissolved inorganic nitrogen (principally nitrate) traversed nearly conservatively from the aquifer through shallow estuarine sediments to coastal waters at flux rates of 1–3 mmol m−2 h−1. A significant relationship between pore water NO3-N concentrations and NO3-N flux rates may provide a rapid method of estimating nitrogen loading from groundwater to the water column.

Use of geoelectrical methods in groundwater pollution surveys in a coastal environment

Abstract The pollution of coastal aquifers by old landfills can contaminate valuable and scarce water resources in the freshwater lens utilized seasonably by overcrowded communities. The pollutants will ultimately flow into the sea where they may also cause a coastal water pollution problem. We have detected pollution in the freshwater lens from a sanitary landfill near Provincetown, Cape Cod, using the geoelectrical resistivity method. This survey included Schlumberger geoelectrical depth soundings and a horizontal geoelectrical profile using the Wenner configuration. The geoelectrical survey was conducted at a site along Highway 6 where it passes the coastal town of Provincetown and a sanitary landfill that has been in operation since 1954. The depth soundings suggest the characteristic decrease in resistivity vs. depth from the high resistivity of the unsaturated zone to the low resistivity of the saltwater saturated zone. The freshwater lens is clearly identified by the change in slope of the steeply dipping curve of resistivity versus electrode spacing. Interpretations made using a multilayer program, Geomate , resulted in layer resistivities between 460 and 95 ohm·m for the freshwater lens. A comparison with well water resistivities suggests that a layer resistivity of 230 ohm·m or lower is indicative of pollution in the freshwater lens. The results of the geoelectrical depth soundings were confirmed in the Wenner horizontal profile. Both measurements suggest that the pollutants do not spread evenly as one would expect for a homogeneous and isotropic medium. Instead, a preferred channel for the flow of the pollutants is observed along a path from the landfill toward the shoreline. The depth to the saltwater/freshwater interface or, more specifically, to the low resistivity-high resistivity interface appears to be shallow where the freshwater lens is polluted. This was confirmed by pore water well samples that were highly mineralized. The equilibrium postulated by the Ghyben-Herzberg relation appears to be disturbed in the area of aquifer pollution. This rise in the conductivity boundary is caused by the highly mineralized bottom of the contaminant plume that submerges into the saltwater saturated zone. In the area of high freshwater pollution the groundwater can be subdivided into three layers that show a decrease in resistivity with depth. The formation factor, F , defined as the ratio of bulk aquifer resistivity to pore water resistivity, shows unusually high values between 10 and 12. These high values are unexpected for an unconsolidated sand. Pollution residues are suspected to clog the pores and thus to increase the resistivity. It is possible that iron-oxidizing bacteria and the precipitation of dissolved iron or organic pollutants are the cause of the high values of F . If proven correct, these interesting possibilities could lead to future new applications of the geoelectrical resistivity method in contaminant hydroloy.

Surface electrical resistivity in coastal groundwater exploration

Abstract Because of its potential to detect changes in pore-water salinity the surface electrical resistivity method can be a valuable aid in coastal groundwater exploration and investigations. It is essential, however, that the resistivity interpretation be consistent with a hydrogeological model reflecting the fresh-water-salt-water relationship of coastal aquifers. In the electrical resistivity interpretation of phreatic aquifers it should be recognized that the lower boundary of the unsaturated zone corresponds to the top of the capillary zone, not to the water table, and that the lower boundary of the fresh-water layer corresponds only approximately to the top of the fresh-water-salt-water transition zone. The existence of a fresh-water layer can be ascertained qualitatively by visual inspection of electrical sounding curves, provided there is a fresh-water/unsaturated layer thickness ratio of at least four. Good interpretative methodology using an appropriate coastal hydrogeological model can enable the extent of the fresh-water layer to be quantified, but it is not possible to quantify the thickness of the transition zone by geoelectrical interpretation because of suppression effects.

The use of geoelectrics and test wells for the assessment of groundwater quality of a coastal industrial site

Abstract Using geoelectrical depth soundings, potential groundwater pollution was assessed on a former shipyard of the Newport Naval Base, Rhode Island, USA. Besides an increase of the total dissolved solids (TDS), toxic organic compounds and metals were expected in the saturated zone. Two nested test wells were used to support the geoelectrical interpretation and a third one was located in a poorly accessible place. Non-invasive and cost-effective geoelectrics is representative for a sizeable volume, but the field data are non-unique with respect to the interpretation. Test wells, on the other hand, provide precise data, but they are laterally confined, so that a combination of both methods optimizes the results. This survey concentrates on the freshwater-saturated zone of glacial sediments that range in thickness between 1 and 15 m. The sediment is a poorly sorted till that is described as silty, gravelly sand with a low hydraulic conductivity of considerable heterogeneity. The major effort concentrated on a 12-m wide strip located between a sheet pile bulkhead along the shore and a building in which chemicals were stored and handled. This strip was the only place where depth soundings with a depth penetration extending below the freshwater-saturated zone were logistically possible. Other places were unsuitable because of conductive constructions at the surface, concrete pavements, and adjacent buildings. In spite of poor data quality, four of the five geoelectrical depth soundings were identified to be of QH-type ( ρ 1 > ρ 2 > ρ 3 ρ 4 ), with the following hydrogeological correspondence: unsaturated-, freshwater-saturated, seawater-saturated zone, and non-conductive tight glacial till and shale bedrock. The target of this investigation is the freshwater-saturated zone, which is qualitatively recognized on four of the five sounding curves. To avoid an underestimation of the interpreted layer resistivities, the lateral effect from the seawater and iron sheet pile bulkhead was corrected. It was approximated with a semi-infinite vertical boundary between the aquifer and a conducting medium using the image method. Low aquifer resistivities are usually associated with potential pollution in terms of high concentrations of TDS. A semi-quantitative attempt is described to correlate bulk layer with pore water resistivity and finally with equivalent TDS. Because of the presence of clay-sized particles in the sediment, the bulk electrical conductivity is caused by water conduction through the pore channels and the surface conductivity of the particles. Lab sample measurements of resistivity on samples recovered from the test wells provided a calibration curve for the bulk conductivity–water conductivity relationship. In spite of complications due to surface constructions and the heterogeneity of the freshwater-saturated till, it was possible to identify high conductivity groundwater with a conductivity on the order of 3000 μS/cm that is equivalent to approximately 1950 ppm TDS. It is not possible to determine whether or how much of the TDS originates from seawater overwash and how much is attributable to shipyard activities. Test wells, however, showed objectionable amounts of dissolved metals and adsorbed toxic organic compounds.

Self-Assembled Monolayers of Thiophenol on Gold as a Novel Substrate for Surface-Enhanced Infrared Absorption

A unique method of obtaining surface-enhanced infrared absorption (SEIRA) spectra for chemicals that will not chemically attach to a metal surface has been investigated. Surface enhancements are greatest for molecules that bind to metals. In order to achieve greater enhancement for those analytes that do not bind to SEIRA metals, we have investigated self-assembled monolayers as a means of linking analytes to a gold substrate. Monolayers of thiophenol were formed onto sputter-coated gold–silicon substrates. Analytes were deposited onto the thiophenol-coated gold–silicon wafers, and external reflection SEIRA spectra were then measured. Enhancement factors as high as 30-fold compared to those for conventional SEIRA substrates are demonstrated. The self-assembled monolayers on gold substrates are shown to change both relative intensities and band positions of the adsorbed analyte. These intensity changes and frequency shifts show strong interaction of selected analytes with the self-assembled monolayer. This study of phthalates and nitrosubstituted aromatic compounds demonstrates the usefulness of the technique. Spectral changes evident through the use of the thiophenol monolayer are discussed.

Preliminary Observation of Complex Salt‐Fresh Water Mixing in a Beach Aquifer

Continuous observations of beach groundwater salinity over a 35-d period from a monitoring well established in the intertidal zone of a coastal harbor provided intriguing data on the interaction in the intertidal zone between the salt and fresh groundwater. During the monitoring period of the study, both semidiurnal variations and longer temporal trends in groundwater salinity were observed. The semidiurnal salinity variations were observed to occur nearly synchronously, but inconsistently with the tides. However, the salinity relationship with the tides was more complex, switching back and forth from being in-sync (higher salinities at high tide) to out-of-sync (higher salinities at low tide) a total of four times during the 35-d test period. The longer temporal trends showed chloride concentration (representing salinity) varying from as low as 50 mg/L to as high as 3600 mg/L over a period of between 9 to 12 d. The observations from the monitoring well reveal a complex pattern likely resulting from a combination of tidal pumping, density-induced convection, and changes in the terrestrial hydraulic gradient. However, these observations are based upon data from only one monitoring well, and are speculative at this point. A more thorough study of the complex fresh water-saline water relationship in the intertidal zone seems to have merit.